The present invention relates generally to magnetic disk drives, and more specifically to magnetic head sliders for positioning a magnetic transducer over a hard disk surface.
The typical magnetic head slider of current hard disk drive products utilizes two or more coplanar rails or pads for the load developing surface, together with a compression inlet (such as a taper or step) on at least two of the rails for maintaining a positive pitch attitude between slider and disk surface. An external load force is applied to the slider (toward the disk). The gas film pressurization between head and disk balances the external force at some clearance between head and disk. This clearance is called the "flying height" or "fly ht." Of particular interest is the fly ht. at the magnetic recording gap which is usually located near the slider trailing edge in one of the rails. In order to maximize the amount of data stored on a disk surface, it is desired to fly at a constant fly ht. across the disk recording surface. Conventional taper-flat sliders which are currently used in most disk drive products do not fly at a constant height over the recording surface due to the air bearing characteristics of that geometry.
The trend is toward smaller, more portable applications for data storage products. This results in smaller disk drives. The recording head is typically positioned over the recording area of the disk by a rotary actuator. The varying angular orientation of the slider longitudinal axis relative to the disk motion due to the rotary actuator causes the fly ht. and slider roll angle to vary over the recording surface. This angular orientation is referred to as the "skew angle." The skew angle is typically defined to be negative when the slider leading edge is rotated out, away from the disk center. Most disk drives operate in such a way that the sliders start and stop in contact with the disk surface. The relative motion between slider and disk at start-up produces the self-acting gas bearing effect which causes the slider to lift-off and proceed to the fully flying orientation. Intermittent contact occurs between slider and disk as rotation starts until a stable gas bearing is developed. The amount of contact and wear that occurs between slider and disk during rotational start-up depends on the magnitude of load force and how rapidly the gas bearing is developed.
The slider load force that is used in most disk drive products today varies between about 4 and 10 grams. There is, however, a trend toward lower load forces. The increased data densities of new products require a thinner magnetic layer on the disk surface as well as a thinner protective overcoat on top of the magnetic layer. In order to minimize the wear that occurs at the slider/disk interface, there is an increased emphasis on decreasing the average contact pressure between slider and disk. The average gas film pressure may be given by P=F/A, where "F" is the externally applied force and "A" is the total gas bearing surface area of the slider. It is seen that the external force may be decreased and/or the slider gas bearing surface area can be increased in order to decrease the average gas film pressure. Note that this average gas film pressure is also the average contact pressure when the disk is stationary, assuming the entire slider gas bearing surface area is in contact.
As recording densities increase, the gas bearing fly ht. decreases. Current high performance products fly at about 4 microinches and the general trend is toward ever lower values of flying height. In order to develop lower fly ht., the slider rail width or pad width is typically decreased (with the external force held constant). This results in a decreased gas bearing surface area. Another approach is to increase the external force while keeping the slider rail width constant. Note that both of these approaches produce an increase in both the average gas film pressure and the average contact pressure between slider and disk. A list of desirable slider air bearing characteristics for the head/disk interface of near-term hard disk drives is given below:
(1) Low, constant fly ht. over the data band, with very little slider roll PA0 (2) Low slider load force (for decreased wear) PA0 (3) Rapid slider take-off during rotational start-up PA0 (4) High gas bearing stiffness (for increased flying height stability and control) PA0 (5) High gas bearing damping (for increased stability and minimum settling time after access motion) PA0 (6) Acceptable flying characteristics with large skew angle variations over the data band (Large skew angle variations are important in order to increase the width of the data band for increased storage, or to decrease the rotary arm size and inertia for decreased energy requirements during access motion).
Different slider geometry types have different gas bearing characteristics. The taper-flat (TF) type slider utilizes 2 or more rails for developing a positive load. The fly ht. of the TF slider is quite sensitive to the skew angles produced by a rotary actuator. This type slider is relatively simple to manufacture and is utilized in most disk drive products today. The TF slider is described in U.S. Pat. No. 3,823,416 to Warner, the complete disclosure of which is incorporated herein by reference.
A negative pressure (NP) type slider utilizes the positive load developed by longitudinal rails and the negative (subambient) load developed by a recess to provide a net low load. This results in the low net load (adjustable to any value), high gas bearing stiffness, and rapid slider take-off. The conventional NP slider has poor damping characteristics, and provides a flying height and roll angle that is sensitive to skew angle.
In our U.S. Pat. No. 3,855,625 ('625), the complete disclosure of which is incorporated herein by reference, an NP slider, illustrated in FIGS. 1A and 1B is disclosed. The intended application was with a zero skew angle (linear actuator). In such a case, the changing disk velocity with radius has an influence on the positive load developed by the rails which is about in the same proportion to that for the negative load produced by the recess. This results in a near constant flying height over the data band when the skew angle is maintained at near zero. Pressure profiles over the NP slider at zero skew are shown in FIG. 2 and are seen to be nearly symmetric about the slider longitudinal centerline. These and other pressure profiles which will be described later are based on simulations done with a computer code named BOLTZMANN1, which I authored. In these figures, curves A-F represent pressure levels in the transverse direction at various distances from the leading edge of the slider, with curve A at 9% of the slider length, curve B at 36% of the slider length, curve C at 40% of the slider length, curve D at 58% of the slider length, curve E at 76% of the slider length and curve F at 94% of the slider length from the leading edge. These simulations were made on a "70%" size slider which flies on an air film over a 3.5 inch disk spinning at 5400 rpm.)
When this NP slider is used with a rotary actuator, however, the skew motion across the slider causes the low subambient pressure of the recess in a conventional NP slider to be convected over the positive pressure rails. This causes extreme pressure distortion over the rails and causes the slider fly ht. and roll angle to vary unacceptably over the data band. Pressure profiles for the NP slider are shown in FIG. 3 for the case of +10.degree. skew. In this case, the transverse motion due to skew is from right to left on the figure. It may be noted that the subambient pressure of the recess is convected over the left side pad. Other NP type sliders are described in U.S. Pat. No. 4,218,715; U.S. Pat. No. 4,475,135; U.S. Pat. No. 4,420,780 and U.S. Pat. No. 5,062,017, which are incorporated herein by reference. Sliders of each of these patents suffer from flying height sensitivity to skew angle.
Since the NP slider depends on a combination of positive and negative load to provide the net low positive load, the positive pads are wider than those of a conventional TF slider. The increased stiffness of the NP slider is achieved by the larger positive load of the longitudinal rails. The subambient load of the recess has little influence on the overall slider gas bearing stiffness. The increased stiffness of the NP slider contributes to a faster take-off during start-up, and the low net load and wider pads produce a reduced average gas film and contact pressure.
The side boundaries of the recess of the NP slider in our '625 patent are formed by the side edges of the outside rails. In U.S. Pat. No. 4,636,894 ('894), which is incorporated herein by reference, Mo describes the possible problem of controlling the side boundaries of the recess (and thus, the rail width) when using an etching process to form the recess. In '894 (see FIGS. 4A and 4B), Mo suggests the use of a groove and buffer pad to separate the side edges of the recess and rails so as to reduce the sensitivity of the resulting rail widths to the etching process used. The resulting NP slider of the '894 patent has a large flying height sensitivity to skew angle which is similar to that of the TF slider. This flying height sensitivity to skew angle of the '894 patent NP slider is caused by pressure distortion and dilution over the outer rails as flow enters the gas bearing across the rail side edges. Flying height profiles for an NP slider possessing the '894 patent configuration are given in FIG. 5. Two skew distributions, (-10.degree.,+10.degree.) and (0,+10.degree.), are presented. In both cases, there is a wide variation of flying height over the recording zone of the disk.
A further slider type is the TPC slider described in my U.S. Pat. Nos. 4,673,996 ('996) and 4,870,519 ('519), which are incorporated herein by reference. Each slider rail utilizes a transverse pressure contour (TPC) on at least one side edge. The result is that the gas bearing effect of the changing velocity over the data band is almost exactly offset by that of the changing skew angle, producing a constant flying height and nearly zero roll angle.
A combination TPC/NP type slider was also described in '996 and '519, illustrated in FIGS. 6A and 6B. In both TPC patents, the negative pressure recess side boundaries were formed by the slider outer rails and the rails were connected by a cross bar. An objective of the present invention is to provide a new combination low load TPC/NP type slider with even more rapid take-off and with the ability to negotiate even wider skew angle variations with constant flying height.